Phosphoric acid is generally produced in accordance with two processes. In one process, phosphoric acid is produced from phosphate ore by contact with a strong mineral acid, such as sulfuric acid. Calcium in the phosphate ore is generally separated in the process as gypsum whereas the other constituents in the phosphate ore remain in the phosphoric acid as dissolved solids or soluble salts to the extent they cannot form slightly soluble compounds in the strongly acid medium with either sulfuric or phosphoric acid. In many instances, these impurities interfere with further processing of the phosphoric acid, and often the employment of the technical grade acid for many purposes is rendered impossible by these impurities. Efforts have not been lacking to free the phosphoric acid of the impurities to permit a breader utilization of the purified phosphoric acid. In one long-used purification method the phosphoric acid is neutralized with alkalis. Accordingly, the impurities dissolved in the raw acid are precipitated as slightly soluble salts or metal hydroxides. However, this method is suitable only if the phosphoric acid is to be used in the alkali salt form.
If compounds other than alkaline phosphates are to be mde from the phosphoric acid, then purification methods are required whereby the free acid remains. In German Pat. No. 648,295 there is described a method of purifying raw phosphoric acid by treating the phosphoric acid with an organic solvent in the presence of an alkaline compound whereby ionic impurities such as calcium, iron, aluminum, chromium, vanadium and fluoride ions are removed as a viscous mass. After the difficult separation of the viscous mass, phosphoric acid is recovered by distillation of the organic solvent. Mentioned as organic solvents are methyl and ethyl alcohol, and acetone. Alkaline compounds used in such process include free alkali and ammonium bases, the salts of sulfuric acid, oxalic acid, carbonic acid, nitric acid and the like. The amount of alkaline compound to be added is optimally set at 10 to 15% with respect to the raw phosphoric acid. It is clear that the process is only applicable to the purification of phosphoric acid since a partial desaturation of the phosphoric acid is achieved as a result of the large amounts of alkaline compounds required for purification.
In accordance with co-pending application Ser. No. 182,534 filed Sept. 21, 1971, assigned to the same assignee as the present invention, the aforementioned impurities are separated as a flaky, readily filterable precipitate by adding 0.06 to 0.12 moles of alkali or ammonium ions per mole of P.sub.2 O.sub.5 together with the addition of organic acid in an amount of from 2 to 5 times the quantity of phosphoric acid. A portion of the alkali or ammonium ions remained dissolved in the phosphoric acid/organic solvent phase and depending on the concentration of phosphoric acid and amount of solvent added, of from 50 to 70% of such alkali or ammonium ions added may remain in the recovered purified phosphoric acid. Accordingly, the alkali or ammonium ions are removed by passing the phosphoric acid/organic solvent phase through a strongly acidic cation exchanger in the H+ form prior to distillation of the organic solvent. The concentration of iron and chromium ions remaining in the purified acid are in the order of from 0.5 to 1% as compared to the initial concentration thereof.
A number of other purification processes have been disclosed in which the impurities are separated by extraction techniques. All of such extraction processes extract the phosphoric acid by treatment with a water-insoluble or limitedly water-soluble organic solvent to transfer the phosphoric acid and in part also impurities, depending on the distribution coefficient, into the organic phase. By reverse extraction and distillation, respectively, a solvent-free phosphoric acid highly depleted of impurities is recovered. Organic solvent that are insoluble in water or miscible to a limited degree include butanols, amylalcohols, ketones, ethers, esters such as trialkylphosphate, butyalcetate, amylacetate, etc. Organic amines have also been proposed as extraction agents and in some instances in combination with the aforementioned organic solvents.